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Relypsa (RLYP) scored a big win Friday after its chief competitor AstraZeneca (AZN) - Get Astrazeneca PLC Sponsored ADR Report was walloped by a U.S. regulatory blowup.

The U.S. Food and Drug Administration rejected AstraZeneca's potassium-lowering drug ZS-9, the company said Friday. That's terrible for AstraZeneca, which acquired the drug via a $2.7 billion acquisition of ZS Pharma last year.

But ZS-9's setback is fantastic news for Relypsa, which secured FDA approval for its own potassium-lowering drug Veltassa last October. With AstraZeneca out of the game for now, Relypsa has the commercial market -- with peak sales forecasts of about $1 billion -- to itself.

Relypsa shares closed Thursday at $16.60. In premarket trading on Friday, the stock was up 41% to $23.47.

Veltassa is approved to treat hyperkalemia, a condition in which potassium levels in the blood become abnormally high, usually due to poorly functioning kidneys or as a side effect from medicines taken by people to manage heart failure and chronic kidney disease. Elevated potassium can cause abnormal heart rhythm and in worse cases, sudden death.

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AstraZeneca intends to try a second time to secure ZS-9's approval from FDA, also for the treatment of hyperkalemia, but the company offered no timeline Friday for when that might happen. The FDA rejected ZS-9 because of undisclosed manufacturing issues which need to be fixed, the company said, adding that regulators did not ask for additional clinical trials to be conducted on ZS-9.

Until Friday, Relypsa shareholders had not benefitted much from the Veltassa approval. The stock fell to $13, an all-time low, when the drug was approved last October because of an FDA-mandated black-box safety warning in the label cautioning doctors about the potential risk Veltassa could interfere with other drugs hyperkalemia patients might be taking. This week, Relypsa submitted new clinical data to FDA seeking to loosen or remove that safety warning.

The stock rebounded strongly into the end of 2015 but recurrent investor concerns about the pace of the Veltassa commercial launch (Sanofi (SNY) - Get Sanofi Report assists with marketing duties) coupled with the competitive threat from ZS-9 kept Relypsa shares under prolonged pressure.

But FDA rejected ZS-9 and it could take AstraZeneca into next year to get the drug approved. With the near-term competitive threat from ZS-9 gone, Relypsa has the opportunity to stake out a commanding lead with Veltassa before AstraZeneca can even get to the starting line.

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.